A. Field of Invention
In the following description, the term `pacemaker` is used generically to cover implantable antitachycardia devices as well as permanent, temporary and/or external pacemakers and implantable cardioverter defibrillators (ICD's) used to detect cardiac arrhythmia and fibrillation and, optionally, to provide appropriate therapy, if required. The subject invention pertains to pacemakers, and more particularly to a pacemaker with means for discriminating atrial fibrillation from ventricular tachycardia and/or other supraventricular tachycardia through a novel technique by monitoring ventricular rate stability.
B. Description of the Invention
Patients with pacemakers may experience cardiac arrhythmia which may be due (among other causes) to atrial fibrillation or ventricular tachycardia. Atrial fibrillation (AF), while uncomfortable and having deleterious long term effects does not constitute an immediate danger, by itself, to the patient and accordingly, the normal clinical approach to such a condition is to ignore it or to treat it in what is, in many cases, an inadequate manner. For example, AF is often treated by administration of various drugs. However, these drugs have side effects, may not control ventricular rate satisfactorily and typically prevent the recurrence of AF for only a limited time.
One indirect result of atrial fibrillation in patients with intact AV nodal conduction may be a relatively high and irregular ventricular rate. In other words, during atrial fibrillation, the ventricular intervals may vary substantially from one ventricular event to the next. The mechanism for this biological phenomenon is not understood completely. It has been suggested that, during atrial fibrillation, the AV node receives numerous successive stimuli originating from the atrium, and while each stimulus alone has a low amplitude which is insufficient to trigger a ventricular contraction, they do cause partial depolarizations. The effects of these partial depolarizations is cumulative, so that when a sufficient number of such stimuli are received, the AV node is depolarized resulting in unstable random ventricular contractions. (See R.J. Cohen et al, QUANTITATIVE MODEL FOR VENTRICULAR RESPONSE DURING ATRIAL FIBRILLATION, IEEE Transactions on Biomedical Engineering Volume 30, pages 769-782(1983)).
Ventricular instability is undesirable because it is uncomfortable for the patient and causes compromised hemodynamic parameters. Presently, symptomatic patients are treated with drugs which are frequently ineffectual and/or have undesirable side effects or they are treated with AV nodal ablation, a drastic procedure which causes the patient to become pacemaker dependent. However, stabilizing the ventricular rate during atrial fibrillation can lead to improved cardiac output, diastolic blood pressure, pulmonary artery pressure and end diastolic mitral valvular gradient. (C-P Lau, Leung, C-K Wong, Y-T Tai, C-H Cheng. A NEW PACING METHOD FOR RAPID REGULARIZATION OF RATE CONTROL IN ATRIAL FIBRILLATION, Am J Cardiol 65:1198-1203, (1990)).
It has been found that during atrial fibrillation, the ventricle can be stabilized at a rate approximately equal to the average intrinsic ventricular rate. Again, the mechanism of how the ventricle is stabilized by a pacing rate lower than the maximal intrinsic ventricular rate is not completely understood. It has been suggested that ventricular pacing eliminates or decreases the slope of the spontaneous depolarization phase of the AV node. F. H. M. Wittkampf, M. J. L. DeJongste, RATE STABILIZATION BY RIGHT VENTRICULAR PACING IN PATIENTS WITH ATRIAL FIBRILLATION. PACE 9:1147-1153 (1986). F. H. M. Wittkampf, M. J. L. DeJongste, H. I. Lie, F. L. Meigler. EFFECT OF RIGHT VENTRICULAR PACING ON VENTRICULAR RHYTHM DURING ATRIAL FIBRILLATION, J Am Coll Cardiol 11:539-545, (1988). These articles disclose that the ventricle may be stabilized by pacing approximately 95% of the ventricular depolarizations. However this method would cause ventricular pacing during atrial flutter and atrial fibrillation with a regular ventricular response which would not be beneficial and result in overpacing.
Another procedure (see Lau, supra) that was investigated to stabilize ventricular instability during atrial fibrillation was to apply an additional stimulus at a preselected interval after every sensed conducted ventricular beat. The average interval was about 230 ms. However it is believed that this procedure is unsatisfactory because it may result in pro-arrhythmia by pacing the ventricle during the vulnerable period of ventricular repolarization, and moreover, it is not an appropriate technique for arrhythmia classification.
The above-mentioned co-pending application Ser. No. 347,279 discloses an apparatus and method for stabilizing the ventricular rate by gradually increasing the pacing rate until stability is achieved. The apparatus and method also allow the pacing rate to decrease once stability is achieved to account for changing physiological conditions. The present disclosure pertains to an apparatus and method using a similar technique to interpret a relatively fast ventricular rhythm, i.e., to determine whether a cardiac arrhythmia is due to atrial fibrillation or some other tachycardia (i.e., ventricular, sinus, etc.).
In an article (Jenkins, J., Noh, K. H., Bump, T. et al. "A single atrial extra stimulus can distinguish sinus tachycardia from 1:1 paroxysmal tachycardia" Pace 9:1063-1068, 1986), an algorithm is described which made use of interactive pacing to classify arrhythmia. However this technique uses interactive atrial pacing, and therefore, would not be useful for atrial fibrillation treatment because of its inability to capture the atrium during atrial fibrillation. Moreover, this technique cannot be used to diagnose atrial fibrillation.
Other known techniques for cardiac arrhythmia classification include very accurate atrial rate and variability measurements, which require atrial sensing; atrial and/or ventricular morphology analysis, requiring complicated and sophisticated data processing analysis; analysis of the paced depolarization integral (PDI) in the atrium requiring atrial pacing and sensing electrode(s) and capture detection software. Even if available, atrial sensing for cardiac arrhythmia classification can be difficult because these signals often have a very low level and accordingly are hard to amplify and separate from noise, thereby confounding rate and morphology interpretation.
Another disadvantage of known techniques for atrial arrhythmia classification is that they all require an atrial sensing lead. However, the physician may not want to implant such a lead because it may be too difficult and time consuming, and/or the vein may be too small for such an atrial lead.